The present invention relates generally to a gas sampling system, and specifically to a gas sampling system for transporting a hazardous process gas from a supply thereof to a remotely located mass spectrometer.
Mass spectrometry is an analytical method widely used to determine the atomic weight and structure of molecules. The basic technique is carried out at extremely low pressures, and consists of bombarding a gas-phase sample with an electron beam strong enough to fragment the molecules into their respective ions, after which the gas phase ions are separated and analyzed according to their mass-to-charge ratios (m/e). This technique is practiced using a mass spectrometer, which typically consists of five main components: a vacuum system; an inlet into which the sample is introduced; an ion source which separates molecules into their respective ions; an analyzer which separates those ions based on their mass-to-charge ratios; and a detector.
Mass spectrometers are often used to provide real-time analysis of process gases formed during a chemical reaction. However, the mass spectrometer employed is typically located at a remote distance from the process apparatus, typically at a distance of six feet or more. Therefore, a quantity of the process gas must be withdrawn from the process apparatus and transported to the mass spectrometer by way of an inlet system.
Conventional inlet systems accomplish this task by transporting a bulk quantity of sample process gas to the mass spectrometer. At or near the mass spectrometer, an aliquot of the bulk sample is removed and fed into the mass spectrometer. The remaining sample gas is then either returned to the process apparatus, or is simply directed to an exhaust stack, incinerator or the like.
However, conventional inlet systems are inadequate for transporting hazardous process gases (i.e.: gases which are toxic or explosive), because conventional inlet systems withdraw large quantities of the process gas from the process apparatus. The risk of leakage from the inlet system is ever-present. If the process gas is toxic or possibly explosive then possible leakage is unacceptable. Further, the mass spectrometer employed is usually located at a remote distance from the process apparatus, typically a distance of 6 feet or more, further increasing the risk of leakage.
One answer to the above problem is to withdraw smaller amounts of gas, but such conventional inlet systems are inadequate for transporting low flow rate process gasses on the order of less than 10 mL/min. Conventional inlet systems require a process gas flow rate of approximately 60 to 100 mL/min in order to maintain the internal pressure of the mass spectrometer, and to provide a sufficient amount of material to obtain a signal from the instrument. However, where the flow rate of the process gas is on the order of 1 to 5 mL/min, conventional inlet systems cannot be used.
Therefore, it is a first object of the present invention to provide an apparatus for safely transporting a hazardous process gas to a mass spectrometer.
A second object of the present invention is to provide an apparatus for transporting a low flow rate process gas to a mass spectrometer which is located at a remote distance from the process apparatus.
A third object of the present invention is to provide an inlet system which eliminates the need for a process gas return line from the inlet system to the process apparatus.
A further object of the present invention is to provide an inlet system which withdraws an aliquot from process apparatus rather than a gas stream introduced into the inlet system.
Another object of the present invention is to provide a heating device for substantially preventing condensation of the process gas within the inlet system.
Yet another object of the present invention is to provide an inlet system that is easy and economical to manufacture, yet durable.
Another object of the present invention is to provide a method for readily calculating the requisite length and diameter for the conduit which transports the gas from the process apparatus to the mass spectrometer.
The above-listed objects are met or exceeded by the present gas sampling system for transporting a low flow rate process gas from a remotely located process apparatus or other supply thereof to a mass spectrometer. The gas sampling system includes a capillary tube having a predetermined capillary length and capillary diameter in communication with the supply of process gas and the mass spectrometer, a flexible tube surrounding and coaxial with the capillary tube intermediate the supply of process gas and the mass spectrometer, a heat transfer tube, and a heating device in communication the heat transfer tube for substantially preventing condensation of the process gas within the capillary tube.